H125-03
Differential heating and Earth rotation modify lake warming under ice

Friday, 11 December 2020: 17:38
Virtual
Cintia Ramon, EAWAG Swiss Federal Institute of Aquatic Science and Technology, Department Surface Waters - Research and Management, Duebendorf, Switzerland, Hugo Ulloa, Swiss Federal Institute of Technology in Lausanne, Lausanne, Switzerland, Tomy Doda, EAWAG Swiss Federal Institute of Aquatic Science and Technology, Department Surface Waters - Research and Management, Kastanienbaum, Switzerland, Kraig B Winters, University of California San Diego, Scripps Institution of Oceanography, La Jolla, CA, United States and Damien Bouffard, EAWAG Swiss Federal Institute of Aquatic Science and Technology, Department Surface Waters Research & Management, Kastanienbaum, Switzerland
Recorded Presentation
Abstract:
In ice-covered lakes, radiatively-driven convection becomes the main driver of water motion in late winter once sunlight is able to penetrate the ice and heat the water below it. The resulting circulation and mixing determine the spatial distribution of heat within the lake and affect the heat budget of the ice cover. According to the classical one-dimensional understanding of radiatively-driven convection, convective plumes impinge on the stratified layer below, entraining water into a convective mixing layer (CML) that becomes thicker and warmer in time. However, as the CML deepens in the lake interior, regions in the lake that become shallower than the base of the CML heat faster and drive horizontal density gradients. If this excess heat is advected towards the lake interior, the warming and deepening of the CML there speeds up. However, could this horizontal transport of heat be constrained by Earth rotation? To answer this question, we conducted 3D RANS numerical simulations in which the near-surface waters of an idealized ice-covered lake, with conical and axisymmetric bathymetry, was periodically heated, mimicking the diurnal cycle of sunlight, and using an exponential distribution for the radiative forcing in depth (Beer’s law). By modifying the value of the Coriolis frequency (f) in our simulations, we show that the dynamic response to those horizontal density gradients depends on the local intensity of the Coriolis acceleration. We report two distinctive dynamical regimes, controlled by the Rossby number (Ro), that result in a remarkably divergent distribution of incoming heat beneath the ice. In the ageostrophic regime, Ro O(10-1-100), the circulation in the CML is characterized by a combination of radial —gravity currents— and azimuthal —gyres— motions, and the advection of heat from the shallow littoral region effectively increases the deepening and warming rates of the CML in the lake interior. In the geostrophic regime, Ro ≤ O(10-2), the circulation in the CML occurs preferentially in the azimuthal direction, the dynamics of the littoral and the offshore regions decouple and the excess heat is retained in the littoral region.
<< Previous Abstract | Next Abstract >>